1. Field of the Invention
This invention relates to an engine, and more particularly, an engine construction and a lubrication method for a small four-cycle internal combustion engine which is particularly suitable for the use with portable or transportable power tools.
2. Description of the Related Art
U.S. Pat. No. 7,287,508 to Kurihara and U.S. Pat. No. 7,624,714 to Kurihara et al., which are incorporated herein by reference, disclose a light weight and compact prior art small four-cycle engine construction.
Portable power tools such as line trimmers, blower/vacuums, chain saws are mostly powered by two-cycle internal combustion engines or electric motors. Some transportable power tools such as tiller/cultivators, generators are currently powered by two-cycle or four-cycle internal combustion engines. With the growing concern regarding air pollution, there is increasing pressure to reduce the emissions of both portable and transportable power equipment. Electric motors unfortunately have limited applications due to power availability for corded products, and battery life and power availability for cordless devices. In instances where weight is not an overriding factor such as lawn mowers, emissions can be dramatically reduced by utilizing heavier four-cycle engines. When it comes to power tools such as line trimmers, chain saws and blower/vacuums, however, four-cycle engines pose a very difficult problem. Four-cycle engines tend to be too heavy for a given horsepower output and lubrication becomes a very serious problem since portable or transportable power tools must be able to run in a very wide range of orientations except for generators or tiller/cultivators. For some tiller/cultivators powered by four-cycle engines with vertical power shafts, lubrication also becomes a serious problem since it is difficult to use the same lubrication system as engines with horizontal power shafts.
Therefore, it is an object of the present invention to provide a small four-cycle internal combustion engine having low emissions that is sufficiently light in weight to be carried and/or transported by an operator, which is especially suitable for a hand-held or transportable power tool.
It is a further object of the present invention to provide a small four-cycle internal combustion engine having an internal lubrication system enabling the engine to be run at a wide variety of orientations typically encountered during normal operation, which is especially suitable for a portable or transportable power tool.
It is a further object of the present invention to provide a small lightweight four-cycle engine having an engine block, an overhead valve train and a lubrication system to deliver oil to lubricate the crankshaft chamber throughout the normal range of operating positions, which is especially suitable for a portable or transportable power tool.
It is a further object of the present invention to provide a lubricant return system to return lubrication oil into oil reservoir after lubricating parts in the crankshaft chamber and the overhead valve chamber.
In the previously-discussed prior art applications, circular and scroll-type walls play an important role in the inventions. However, the construction to make these walls is not so cost effective and not so compact. Thus, it is a further object of the present invention to provide a more cost effective and more compact construction.
A breather system is also an important issue within engines. U.S. Pat. No. 4,404,936 to Tatebe et al. discloses a typical breather system for a non-portable utility engine, wherein, by pressure change in the crankshaft chamber due to reciprocating motion of a piston, the breathing gas is sent via a check valve to an air cleaner through a breather pipe, and then inhaled into a combustion chamber. The check valve opens and allows a gas flow from the crankshaft chamber to the breather pipe when the pressure in the crankshaft chamber is higher than that in the breather pipe, and closes and does not allow a gas flow from the crankshaft chamber to the breather pipe when the pressure in the crankshaft chamber is lower than that in the breather pipe. Since the pressure in the breather pipe is almost equal to the atmospheric pressure, a pressure in the crankshaft chamber is kept in a little lower than the atmosphere, so that the function of piston rings is kept normal and undesirable leakage of gas from the combustion chamber to the crankshaft chamber is prevented and, in a stable running condition of the engine, the gas flow in the breather pipe is substantially only a blow-by gas from the combustion chamber.
However, in an engine for a portable or transportable power tool, especially in an engine which lubricant is carried and circulated by the gas flow made by the pressure change in the crankshaft chamber, the circulating gas includes not only the blow by gas but also a fresh air taken from the outside of the engine. For instance, in U.S. Patent application 2011/0067669A1, a breathing gas is inhaled from a valve chamber to an air cleaner and then oil is separated in the air cleaner and sent to a crankshaft chamber through a check valve by reciprocating motion of a piston. At that time, the oil, separated from the breathing gas and sent to the crankshaft chamber, brings fresh air from the air cleaner. Further, since a passage from the air cleaner to the crankshaft chamber is joined with a suction passage of oil from an oil reservoir to the crankshaft chamber, in every reciprocating stroke of the piston, some quantity of fresh air inhaled from the outside of the engine through the air cleaner mixes with the oil suctioned from the oil reservoir and enters into the crankshaft chamber. As a result, suction of liquid oil from the oil reservoir is obstructed by inhalation of fresh air so that gas pressure in the crankshaft chamber is higher than the previously mentioned non-portable engine (for instance, U.S. Pat. No. 4,404,936 to Tatebe) and in some cases, it is more than the atmospheric pressure in the down stroke of the piston. This higher pressure produced in the crankshaft chamber helps to circulate lubricant carried by gaseous mixture, but spoils function of piston rings. Therefore the mechanism described above increases blow by gas from the combustion chamber and increases oil consumption. Further, increase of circulating gas brings an increase of a pumping loss.
Therefore, it is a further object of the present invention to provide a lubrication and breather system which minimizes an increase of a pumping loss due to the increase of the circulating gas.
These objects, features and advantages, and other objects, features, and advantages of the present invention will become apparent upon further review of the remainder of the specification and the accompanying drawings.